Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

Abstract. To gain better insight into the importance of predator and resource control in New Zealand lakes we surveyed the late summer trophic structure of 25 shallow South Island lakes with contrast-ing nutrient levels (6–603 µg TP l–1) and fish densities. Total catch of fish per net (CPUE) in multi-mesh gillnets placed in the open water and the littoral zones was positively related with the nutrient level. Trout CPUE was negatively correlated with total phosphorus (TP) and total nitrogen (TN). Zooplankton seemed largely influenced by fish, as high fish CPUE coincided with low zooplankton and Daphnia biomass, low average weight of cladocerans, low contribution of Daphnia to total clado-ceran biomass, low ratio of calanoids to total copepod biomass and low ratio of zooplankton biomass to phytoplankton biomass. However, chlorophyll a was only slightly negatively related to Daphnia biomass and not to zooplankton biomass in a multiple regression that included TN and TP. Ciliate abundance was positively related to chlorophyll a and negatively to Daphnia biomass, but not to total zooplankton biomass, while no relationships were found between heterotrophic nanoflagellates and zooplankton. The relationships between fish abundance and nutrients and fish abundance and zooplankton:phytoplankton ratio and between chlorophyll a and TP largely followed the pattern obtained for 42 north temperate Danish lakes. We conclude that fish, including trout, have a major effect on the zooplankton community structure and biomass in the pelagial of the shallow oligotrophic to slightly eutrophic New Zealand lakes, but that the cascading effects on phytoplankton and protist are apparently modest

Nutrients and Saltwater Exchange as Drivers of Environmental Change in a Danish Brackish Coastal Lake over the Past 100 Years

Many northwest European lake systems are suffering from the effects of eutrophication due to continued loading and/or poor, ineffective management strategies. Coastal brackish lakes are particularly difficult to manage due to complex nitrogen, phosphorus, and salinity dynamics that may exert varying influence on lake biological communities, but long-term data on how these important and often biodiverse systems respond to change are rare. In this study, palaeolimnological data (including sedimentary parameters, diatoms, and plant macrofossils) and environmental monitoring data (for the last ~40 years) have been used to assess environmental change over the last 100 years in Kilen, a brackish lake in northwest Jutland, Denmark. Kilen has been regularly monitored for salinity (since 1972), TP (from 1975), TN (from 1976), and since 1989 for biological data (phytoplankton, zooplankton, and macrophytes), which allows a robust comparison of contemporary and paleolimnological data at high temporal resolution. The palaeolimnological data indicate that the lake has been nutrient rich for the last 100 years, with eutrophication peaking from the mid-1980s to the late 1990s. Reduced nutrient concentrations have occurred since the late 1990s, though this is not reflected in the sediment core diatom assemblage, highlighting that caution must be taken when using quantitative data from biological transfer functions in paleolimnology. Lake recovery over the last 20 years has been driven by a reduction in TN and TP loading from the catchment and shows improvements in the lake water clarity and, recently, in macrophyte cover. Reduced salinity after 2004 has also changed the composition of the dominant macrophyte community within the lake. The low N:P ratio indicates that in summer, the lake is predominately N-limited, likely explaining why previous management, mainly focusing on TP reduction measures, had a modest effect on the water quality of the lake. Despite a slight recovery, the lake is still nutrient-rich, and future management of this system must continue to reduce the nutrient loads of both TN and TP to ensure sustained recovery. This study provides an exceptional opportunity to validate the palaeolimnological record with monitoring data and demonstrates the power of using this combined approach in understanding environmental change in these key aquatic ecosystems